Titanium metal–organic frameworks for photocatalytic CO2 conversion through a cycloaddition reaction

The elevated levels of CO2 in the atmosphere have been a major concern for environmental scientists. Capturing CO2 gas and its subsequent conversion to useful organic compounds is one of the avenues that have been extensively studied in the last decade. The photocatalytic cycloaddition of CO2 is a promising approach for effective CO2 capture and the production of value-added chemicals such as cyclic carbonates. MOF-901, a titanium-based metal–organic framework with hexagonal layers and imine linkages, was successfully oxidized in this study to MOF-997, incorporating amide linkages using Oxone. Both MOFs displayed remarkable photocatalytic activity in CO2 cycloaddition under mild conditions, including moderate temperatures and visible light exposure. Particularly noteworthy is MOF-997, exhibiting superior performance with donor–acceptor active sites, achieving a 99.9% yield in catalyzing CO2 conversion from styrene epoxide to styrene carbonate under solvent conditions.


Table of Contents
Section S1 Materials and methods

Section S9 References S10
Section S1: Materials and methods

Analytical techniques:
Powder X-ray diffraction (PXRD) was measured using Rigaku Mini-Flex benchtop X-ray diffractometer having CuKα radiation tube (λ = 1.542Å) operated at 40 kV across a range of 3−30° 2θ and a rate of 0.5° min −1 .Fourier-transform infrared spectroscopy (FTIR) was collected using Agilent Technologies Cary 600 series spectrometer.Morphological features of the MOFs were analyzed using a Quattro ESEM fitted with energy dispersive X-ray spectroscopy (EDX) detector that was used to study elemental mapping of the samples.Thermogravimetric analysis (TGA) was carried out using Mettler Toledo instrument TGA-2 analyzer using N 2 airflow and aluminum pan as sample holders.Porosity was determined through N 2 sorption measurements at 77 K using a micromeritics Tri-Star 11 plus gas sorption instrument.UV-Vis diffuse reflectance spectroscopy for the bandgap determination was obtained using Shimadzu UV-3600.Photocycloaddition reactions were conducted using xenon lamp as source of visible light and the experiment was performed in sealed tubes under stirring conditions.Proton nuclear magnetic resonance ( 1 H-NMR) spectra were collected using Varian 400 MHz spectrometer with chloroform-d and dimethyl-sulfoxide-d solvents.Solid-state NMR spectra were collected using a 11.4 T magnet on a Bruker Advance I spectrometer operating at 125.75 MHz for 13 C-NMR.

Synthesis of raw materials
Synthesis of imine linker [3] .The synthesis process began by combining 4-aminobenzoic acid with 1,4-benzene dialdehyde at a 2.1 mole ratio in 2 mL of methanol, followed by a 30-minute evacuation.To this mixture, 2-3 drops of pyrimidine were introduced, and the reaction was left stirring in an ice bath for 18 hours.The resulting yellow solid was washed in methanol, subjected to crystallization, and subsequently characterized using 1 H NMR, FT-IR, and PXRD techniques (Scheme S1). [4].Initially, 1.15 grams (8.37 mmol) of solid 4-aminobenzoic acid dissolved in 15 mL of NMP was chilled to 5°C in an ice bath.Over the course of an hour, 0.812 grams (4.00 mmol) of solid terephthaloyl chloride was cautiously added.The reaction mixture was stirred at 5°C for 2 hours and then allowed to continue stirring at room temperature for an additional 12 hours.Following this, 20 mL of distilled water was added.The resulting product underwent filtration and subsequent washing: initially with 20 mL of NMP, then five washes with 50 mL of distilled water, two rinses with 20 mL of methanol, and finally, it was dried at 65°C for 12 hours to yield 4,4'-(terephthaloylbis(azanediyl))dibenzoic acid (Scheme S1).

Scheme S1. Synthesis and chemical structures of imine and amide linkers that comprise MOF-901
and MOF-997, respectively.
Synthesis of Ti-oxo cluster [5] .Starting with 16 milligrams of 4-amino benzoic acid (0.116 mmol) dissolved in a 2 mL mixture of isopropanol and methanol (1:1), the solution underwent evacuation before the addition of 9 microliters of Titania isopropoxide.This mixture was then transferred into a tightly sealed Teflon-lined autoclave and subjected to an oven at 140 degrees Celsius for 72 hours (approximately 3 days).Post-reaction, the resulting yellow crystals were washed successively with methanol and dimethyl acetamide, followed by activation in dichloromethane for 24 hours.The obtained yellow solid was prepared for further characterization.

Synthesis of MOFs
MOF-901.Was synthesized following a procedure reported in the literature [1] with slight modifications.Each experiment consisted of 16 mg of 4-amino benzoic acid (0.116 mmol) dissolved in 2 mL of isopropanol:methanol (1:1 mL).A mixture of isopropanol and methanol (1:1 mL) was used to dissolve 10 mg of benzene-1,4-dialdehyde (0.074 mmol).Subsequently, 9 L of titanium isopropoxide (0.032 mmol) was added into the 4-aminobenzoic acid solution.The two mixtures were then mixed in a Teflon lined autoclave and tightly capped.The reaction mixture was heated at 140 °C in a preheated oven for 72 hours.The yellowish brown solid, MOF-901, was filtered and washed with methanol, then DMA for 12 hours, followed by Soxhlet extraction for another 24 hours using chloroform.The samples were solvent exchanged by soaking in dichloromethane for 36 hours and the activation was executed under dynamic vacuum at 120 °C for 24 hours.MOF-997.Was synthesized by post-synthetic oxidation of MOF-901.The oxidation was done by using oxone as the oxidant agent.Particularly, oxone (30 mg, 0.112 mmol) and 30 mg of MOF-901 were loaded into a 20-mL vial.DMF (5 mL) and glacial acetic acid (1 mL) were then added to the vial.The solution was stirred for 5 hours at room temperature.The solid was then collected through filtration, washed with sodium thiosulfate (10% aqueous solution), water, and tetrahydrofuran.The oxidized sample was solvent exchanged using Soxhlet extraction with chloroform for 24 hours.The sample was then washed with acetone, methanol, and dichloromethane before being activated under dynamic vacuum at 120 °C for 24 hours.

Section S3: Powder X-ray diffraction
The activated MOFs were loaded onto a holding disc (the sample holder) and placed in the chamber of the X-ray instrument at room temperature.The samples were measured from 3−30° 2θ with a scanning rate of 0.5° min −1 .

Section S4: N 2 and CO 2 Sorption
The solvent-exchanged sample was loaded into a pre-weighted sorption cell.The sample was then activated at 120 °C for 24 hours and then re-weighed before being securely placed in the holding chamber of the sorption instrument.Liquid nitrogen in the flask was added to the stage to ensure the sorption cell within the chamber can be fully immersed.The experiment was initiated and allowed to run until all adsorption and desorption points were completed.Section S5: Thermogravimetric analysis (TGA) Samples, weighing up to 5 mg each, were positioned on alumina sample holder pans.The samples were then heated to 800 °C with a heating rate of 5 °C per minute.

Section S6: Scanning electron microscopy (SEM)
Samples, each measuring 1 cm 2 , were affixed to FESEM stubs using carbon tape and rendered conductive by applying a thin platinum coating with a sputter coater.These prepared samples were subsequently placed in the SEM's sample holding chamber, where various parameters were adjusted for the morphological analysis.Additionally, EDX analysis was conducted using the same instrument to determine the elemental composition percentages in the analyzed samples.

Section S7: UV-Vis diffuse reflectance spectroscopy
To determine the bandgap of MOF-901 and MOF-997, the samples were pressed onto calcium carbonate and loaded into the instrument's sample holding chamber.After adjusting the parameters, a UV-Vis beam was directed at the sample.The data obtained was then used to create plots of absorbance vs. wavelength, and bandgap estimates were calculated using Tauc plots.

Section S8: Cycloaddition reactions of CO 2
Cycloaddition reactions of CO 2 were carried out through a procedure reported in the literature. 2 Different sample weights of MOF-901 and MOF-997 (5 mg, 10 mg, 15 mg, and 20 mg) were prepared and placed in sealed tubes.Each tube was loaded with 9 mg of co-catalyst tetrabutylammonium bromide (TBAB).Subsequently, 54 L of styrene-oxide was added to each reaction tube, along with a magnetic bar.Finally, 3 mg of solid CO 2 was added to each reaction tube, and the tubes were promptly sealed to prevent CO 2 leakage.The reaction samples were transferred to a halogen lamp (380-780 nm wavelength range) with no cut-off filter positioned adjacent to the magnetic stirrers, where the samples were securely placed.The reaction proceeded for 24 hours.The products were obtained by washing the samples with acetonitrile and filtering them using nylon syringe filters.The collected colorless product was dried using a rotavapor at room temperature, operating at 400 rpm for 30 minutes.The dried solid sample was subsequently dissolved in deuterated chloroform for 1 H-NMR analysis.Table S3.Cycloaddition of CO 2 and styrene oxide under neat (green) conditions Entry Photocatalyst [a] Catalyst (mol%) Yield% [b] TON [c] TOF (h -1 )

Figure S4. 1
Figure S4. 1 H-NMR spectra of the styrene carbonate product catalyzed by MOF-997 for three consecutive cycles.

Figure S5. 1
Figure S5. 1 H-NMR of spectra of the styrene carbonate product catalyzed by the amide linker.